Ongoing studies make it clear that the intravenous injection of high doses of Adenovirus (Ad) based vectors offer a tremendous potential for treatment of disseminated cancers, inherited diseases, vascular diseases, as well for gene therapy of the liver for a variety of hepatic/genetic diseases. However, the systemic administration of Ad based vectors is currently viewed as a highly risky maneuver, as significant acute toxicities can be incurred, including chemokine/cytokine release, endothelial cell damage, Kupffer cell activation, hypotension, thrombocytopenia (low platelet counts), and as noted in the Gelsinger tragedy, the systemic inflammatory response syndrome and death. Precious little is known about the innate systems that are immediately triggered by injection of an Ad vector at high doses. Activation of these systems is predicted to be pivotal in initiating and/or modulating subsequent host immune responses, both innate and adaptive. One of these first line innate host defense systems is the complement system. Inappropriate complement activation can result in a number of dire consequences, including anaphylactoid reactions, ARDS (adult respiratory distress syndrome), hypotensive shock, DIC (disseminated intravascular coagulation), cytokine release and systemic inflammatory response syndromes. More specifically, complement activation products released after complement activation (i.e: C3a, C5a) are potent inflammatory mediators, which are known to activate Kupffer cells, as well activate vascular endothelium, leading to neutrophil recruitment, and platelet aggregation. Intriguingly, many of these consequences are also noted after high dose Ad injections. Based upon these considerations, we forward the following Central Hypothesis: "Ad vector interactions with the complement system results in many of the limitations and toxicities typically attributed to the use of Ad vectors". The aims of this proposal will directly test the central hypothesis. The results of this line of investigation will move the field of gene transfer using systemically administered Ad vectors forward.